1. Field of the Invention
This invention relates generally to a stage apparatus, and more particularly to a stage apparatus suitably applied for a precision positioning apparatus for a table on which a work of a semiconductor producing equipment or a precision working machine or a specimen to be measured by a precision measuring instrument is carried.
2. Description of the Prior Art
Operation in a progressively high efficiency and high precision is required in recent years for a stage apparatus employed for semiconductor producing equipments including light exposing apparatus, precision measuring instruments employing a laser beam or precision working machines, that is, for an apparatus for moving a table on which a work or a specimen (hereinafter referred to only as work) such as a semiconductor wafer is carried to position and stop the table at a predetermined position. A stage apparatus of the type mentioned generally forms a peculiar vibration system including a work supporting and guiding mechanism including a table and a driving mechanism for the table. Thus, such stage apparatus has a serious problem in that vibrations produced upon stopping of the table (upon deceleration) deteriorate the positioning efficiency (that is, shortness of a period of time required before a stopped condition is reached) and the accuracy (positioning accuracy with respect to an aimed stopping position). Therefore, improvement in dynamic characteristic of the vibration system is demanded.
Referring to FIG. 7 which schematically shows an exemplary one of conventional stage apparatus, a table 20 for receiving a work thereon is supported and guided for linear movement in a leftward or rightward direction (hereinafter referred to as X direction) in FIG. 7 by a guide mechanism 21 composed of an arrangement of a plurality of needle bearings, and is driven to move by a motor 24 by way of a feed screw 23 which extends in parallel to the passage provided by the guide mechanism 21. A coordinate axis x for driving the table 20 is formed on a base 22, and a current position x of the table 20 on the coordinate axis x is measured precisely by a laser interferometer 25 on the base 22 and a reflector (flat mirror) 26 provided at an end portion of the table 20. A controller 27 controls operation of the motor 24 so that the table 20 is fed to an aimed stopping position x.sub.0 for the table 20 prescribed on the coordinate x in accordance with a result of measurement by the laser interferometer 25 such that it is first accelerated and then moved at a uniform speed whereafter it is decelerated until the current position x measured by the interferometer 25 coincides with the position x.sub.0. The table 20 is positioned with respect to the base 22 in this manner.
Here, however, the table 20 forms a vibration system which is supported by a resilient force of the feed screw 23 and an attenuating force of the guide mechanism 21, and attenuating vibrations are produced on the table 20 by a force of inertia produced upon deceleration and stopping of the table 20. Accordingly, a next operation (for example, a working operation) cannot be started until after such vibrations are attenuated until the table 20 is stopped completely after stopping of the motor 24, and the waiting time for such attenuation deteriorates the efficiency in positioning operation.
Thus, in such conventional apparatus as described above, the mass of a movable part (table 20) is designed small so as to reduce the vibration energy of the vibration system while a lubricant having a high viscosity is employed for the guide mechanism 21 in order to achieve a possible most quick attenuation of vibrations.
Meanwhile, as for an accuracy required here, the feed screw 23 cannot be regarded as a rigid body and a play exists at every joining portion or contacting portion, and consequently, also an attenuating force of lubricating oil and an unnecessary frictional force act. Therefore, a stopping time of the motor 24 does not coincide with a stopping time of the table 20. Consequently, the actual stopping position x.sub.1 of the table 20 does not coincide with the aimed position x.sub.0, and the positioning accuracy is deteriorated by the presence of a difference .DELTA.x between them.
Thus, conventionally the rigidity of the feed screw 23 and so forth is generally designed high and the play and the viscous force and frictional force at every joining portion or contacting portion are selected small so as to improve the responsibility of the table 20 to the motor 24.
By the way, if the responsibility of the table 20 to the motor 24 is raised in order to reduce the difference .DELTA.x as described above, then the attenuating force for vibrations produced upon stopping of the table 20 becomes short, and consequently, another problem takes place that the attenuation waiting time is extended. Or on the contrary if precedence is taken to attenuation of vibrations, then the responsibility of the table 20 to the motor 24 is deteriorated, and consequently, the position difference .DELTA.x upon stopping of the table 20 is increased. Accordingly, when it is intended to cope, only by any of such means as described above, with a progressively increasing demand for improvement in both of the efficiency and accuracy in positioning, there is a certain limitation, which is a serious problem.
Thus, such an attempt has been made to minimize vibrations upon stopping of the table 20 by controlling the motor 24 in accordance with an optimum deceleration curve of the table obtained taking the attenuating force of the guide mechanism 21 into consideration. However, since the needle bearings employed for the guide mechanism 21 include differences in working error or lubricating condition among individual products or over the overall length, the attenuating forces of them present such a large dispersion that precise numeric conversion or normalization cannot be achieved and also actual effects lack in reproducibility.
Thus, also such an apparatus has been proposed wherein an unstable member which provides an acting force lacking in reproducibility such as a needle bearing or a drive shaft is not employed but a table is supported in a non-contacting relationship by means of an air bearing and the table is driven to move by a linear motor provided between a base and the table. With such stage apparatus wherein a table is floating in the air, an actual behavior of the table coincides with a high degree of accuracy with a table deceleration curve obtained in accordance with an expression. However, it does not provide a sufficient thrust. Besides, since a mechanical vibration attenuating force including a frictional force does not act at all, it is unstable and lacks in fixed position holding faculty such that fine vibrations continue for a long period of time.
On the other hand, as for a difference .DELTA.x remaining upon stopping of a table, it has been attempted to construct, for example, the table from two parts including a bolt part held in meshing engagement with a feed screw and main part constituting the table and to provide a fine feeding device such as a piezo-electric element between the two parts such that the table part is finely fed after stopping of movement thereof by the feed screw in order to correct the difference .DELTA.x. With the arrangement, however, the entire table is complicated in structure and increased in weight. Accordingly, the arrangement has such drawbacks that the vibration energy is increased, that the attenuating time is elongated and the controlling procedure is complicated.